I agree with G-Benz' procedure.
I don't even mess with the tensioner, for fear that it will fail to tension when I try to tighten it back up.
I don't use GB's screwdriver method but I pull the belt up over the very top pulley, after it's been set on the far left and right pulleys. The belt length from these two points allows some leverage in getting the belt up onto the last top pulley. (fan clutch?)
Belt has plenty of tension with no slips doing it this way, and I've had no problems with this technique FWIW.
Good luck!